Quality control of a product or process has become a large part of the economics of industry. A major concern of quality control is accuracy in measuring and the ability to detect the slightest fault in a variety of products and processes. Various devices are used to measure differences in weight, temperature and other dimensions. Such devices are usually nonportable, time consuming, inaccurate, invariable for use in detecting more than one object, and often incapable of giving a quantitative analysis.
Radiation detectors can be used to detect abnormalities by measuring temperature change and heat loss or gain. Radiation detectors have been used as a non-contact alternative to many temperature sensors. Infrared scanning devices have also been used to detect temperature differences between a subject and a reference as well as to measure heat loss from machinery, plumbing, electrical lines and the like. Typically such radiation detectors and infrared scanning devices employ radiation sensors which respond to changes in radiation in the order of {fraction (1/10)} second. Such sensors are not only fast, but accurate and economic as operations of interest do not need to be shut down during detection.
Radiation detectors are based on the principle that the thermal radiation emitted from a subject is proportional to the temperature of the subject raised to the fourth power. The radiation emitted is also a function of the emissivity of the subject and of background radiation, but can be calibrated out for applications in which the target has consistent properties.
One type of radiation sensor is a thermopile. Thermopiles in general have been used to provide an indication of target temperature. A thermopile operates on the principle that sensed radiation causes a voltage to be produced at the thermopile output which is indicative of the difference between the hot and cold junctions of the thermopile.
One typical problem with radiation sensors such as thermopiles is their tendency to become overheated by energy trapped within the device. Such overheating and retaining of energy by the radiation sensor causes inaccuracies in the temperature readings. Many sensing applications require close range detection. A user in such a situation often runs the risk of heating or cooling the device with changing environmental conditions, which may change the cold junction temperature of the device or perhaps even distort the sensor output by causing uncontrolled thermal gradients. In addition to heat management problems, radiation sensor devices face dirty as well as harsh environments. Elaborate cooling, purging and cleaning systems have been used, but are expensive, clumsy and require maintaining close calibration.
Provided with the present invention is a radiation detector having a thermopile sensing radiation emitted from a target, and providing an output signal indicative of the temperature of the target. To allow calibration of the thermopile output signal, a calibrator such as a potentiometer or other variable resistance is provided at the thermopile output. By enabling a user to adjust the potentiometer, the thermopile output signal may be user scaled to calibrate the output signal to intersect a thermocouple output response at a desired target temperature.
Although the thermopile and potentiometer together form a detector which can be adjusted to suit a particular application, a preferred embodiment also has a thermocouple which provides an output signal that combines with the output signal of the thermopile to produce a total output signal. To provide compensation for output changes due to changes in local temperature, the change in the thermopile output signal with a change in the local temperature is inversely related to the change in the thermocouple output signal with a change in the local temperature.
By connecting the thermocouple electrically in series with the thermopile, the output voltages of the thermopile and the thermocouple combine to provide a total output voltage. The hot junction of the thermocouple is held at the cold junction temperature of the thermopile. Thus, with the thermopile thermal response to the common junction local temperature being close to the inverse of the thermocouple thermal response to the local temperature, changes in the total output signal are substantially independent of fluctuation of the temperature at which the thermocouple hot junction and the thermopile cold junction are held.
In one embodiment, a lens is provided for filtering out shorter wavelengths from the radiation sensed by the thermopile. This helps improve the linearity of the thermopile thermal response in a target temperature range of interest. With the total output response of the sensor approximating a linear function in a temperature range of interest, a linear output means such as a meter responsive to linear inputs may be controlled directly from the total output signal.
In another embodiment, the filter passes shorter wavelengths, substantially filtering out longer wavelengths such as those greater than 6 microns. Although such a sensor loses linearity, it is significantly less sensitive to changes in emissivity with change in temperature over a narrow target temperature range. Accordingly, such a device is particularly suited to low emissivity targets.
The cold junction temperature to which the hot junction temperature of the thermopile is referenced is at the local hot junction temperature of the thermocouple which is referenced to the thermocouple cold junction. The thermocouple cold junction reference temperature may be located remote from its hot junction and the thermopile sensor. This prevents changes in output of the sensor due to incidental heating of the local reference temperature due to its proximity to the target.
One embodiment of the present invention provides for a differential radiation detector. In that embodiment, a first thermopile senses radiation from a first target and provides an output signal indicative of the temperature of the first target. A first thermocouple provides an output signal which combines with the output signal of the first thermopile to produce a first total output signal. A change in the output signal of the first thermopile with changes in a first local temperature is inversely related to a change in the output signal of the first thermocouple with changes in the first local temperature.
In addition to the first thermopile/thermocouple combination, a second thermopile senses radiation from a second target and provides an output signal indicative of the temperature of the second target. A second thermocouple provides an output signal which combines with the output signal of the second thermopile to produce a second total output signal. A change in the output signal of the second thermopile with changes in a second local temperature is inversely related to the change in the output signal of the second thermocouple to changes in the second local temperature. The cold junction of the first thermocouple and second thermocouple are held to a common temperature and the thermocouple/thermopile pairs are coupled to provide a differential output. Calibrators and lenses may also be provided in the same manner as with the single thermopile sensor embodiment. It is preferable that the thermopiles are matched and the thermocouples are matched to provide an accurate differential response.
In accordance with another embodiment of this invention, a radiation detector has a temperature dependent variable resistor coupled to the thermopile and providing a variable resistance that combines with the thermopile output voltage to produce a linearized thermopile output voltage. As such, the thermopile output, linearized by the thermistor, combines with the linear thermocouple output to provide a detector output that is more stable with changes in the thermopile cold junction temperature.
In the aforementioned embodiments, the thermopile and the thermocouple together form a detector suitable for applications for an expected mean target temperature and within a common junction local temperature range. However, since the linear thermal response of the thermocouple is employed to compensate for the non-linear thermal response of the thermopile, the local temperature range of the common junction must be known and relatively narrow. Accordingly, the primary advantage of this embodiment is that detector output less dependent on the thermopile cold junction temperature over a broad range.
Accordingly, a thermocouple is connected electrically in series with the thermopile/thermistor circuit such that changes in the thermocouple output voltage due to changes in thermopile cold junction temperature are inversely related to changes in the linearized thermopile output voltage due to said changes in thermopile cold junction temperature. Thus, this embodiment utilizes the thermocouple, which provides a linear thermocouple output, to compensate for the linearized thermopile output with changes in the cold junction temperature of the thermopile, thereby maintaining a stable detector output voltage for a given target temperature. Since the thermocouple is connected in series with the thermopile/thermistor circuit, the remote thermocouple cold junction becomes the thermopile reference. As such, there is no need to measure the thermopile cold junction temperature or to force the cold junction temperature into a particular range.
The temperature dependent variable resistor preferably comprises at least one negative temperature coefficient (NTC) thermistor electrically connected in series with the thermopile and thermally coupled to the cold junction of the thermopile. To achieve linearization of the thermopile output voltage over a thermopile cold junction temperature range, an NTC thermistor is selected wherein the change in the resistance of the thermistor due to a change in thermopile cold junction temperature modifies the thermopile output response in a manner that is inversely related to the change in the thermopile output voltage with said change in a thermopile cold junction temperature. In an alternative configuration, at least one positive temperature coefficient thermistor may be electrically connected in parallel with the thermopile and thermally coupled to the thermopile cold junction. In either case, the resulting thermopile output voltage is a more linear function with changes in the thermopile cold junction temperature.
This embodiment of the present invention is particularly useful in applications in which the target temperature is known and relatively stable. Depending on the target temperature range of interest, different types of thermistors or even multiple thermistors may be used in combination with standard resistors to provide for linearization of the thermopile output voltage over a wide range of thermopile cold junction temperature variations.
As in previous embodiments, a calibrator such as a potentiometer may be employed to fine-tune the linearized thermopile output response to intersect a thermocouple output response at a desired target temperature to produce a stable detector output for a thermopile cold junction temperature range of interest. Also, since thermopiles have parameters that vary significantly from device to device, the potentiometer may be adjusted to compensate for these variations such that a number of devices may be tuned to provide the same detector output for the desired target temperature.
In accordance with another aspect of the present invention, the thermocouple may comprise a nonintersecting pair of leads formed of different thermocouple materials and coupled to a thermopile circuit such that the thermopile circuit actually serves as the hot junction of the thermocouple. For the thermocouple to be electrically connected in series with the thermopile, a first thermocouple lead is electrically connected to one of a pair of thermopile leads which are connected to a thermopile circuit and therefore held at the cold junction temperature of the thermopile. Although the second thermocouple lead is not electrically connected directly to a thermopile lead, it is electrically connected to the thermopile circuit. Further, the second thermocouple lead is mounted in close proximity to the thermopile and thermally coupled to the cold junction of the thermopile with epoxy. With both thermocouple leads held at the same temperature, the temperature of the thermopile cold junction, the leads do not have to intersect to provide a thermocouple hot junction.
A meter may be coupled to the detector output. The meter may be of a type typically used to measure a thermocouple output. Since both the meter and the thermopile circuit are high impedance devices, the thermopile acts as an antenna receiving stray high frequency noise which distorts the meter measurement. In accordance with the present invention, a filter is coupled to the thermopile to attenuate high frequency noise, specifically noise at and above 60 Hz. Preferably, the filter comprises a capacitor having a value of 1-5 xcexcf and which is connected in parallel with the detector output. At high frequencies, the capacitor causes the thermopile circuit output impedance to be low thereby eliminating the presence of high frequency noise at the meter.
In another embodiment of the present invention, a radiation detector comprises a thermopile and a thermistor and provides a linearized thermopile output voltage. Since the linearized thermopile output is a linear function with changes in the thermopile cold junction temperature, a linear output means with linear cold junction compensation may be coupled to the detector to provide temperature indications.
In yet another embodiment of the present invention, a temperature monitoring system monitors the temperature of a product positioned in a process chamber. The monitoring system comprises a thermopile which senses radiation emitted by the product and provides a thermopile output signal indicative of the product temperature. Preferably, a thermocouple and a temperature dependent variable resistor are electrically and thermally coupled to the thermopile to provide an output signal indicative of the product temperature. As long as the product temperature remains within acceptable limits, the output signal is a linear function over a product temperature range of interest and is independent of fluctuations in local temperature.
The monitoring system also comprises a thermal heat sink having a first end extending into the process chamber and having a second end disposed in an ambient temperature environment. The heat sink may comprise a copper pipe or a heat pipe. The radiation detector is thermally coupled to the heat sink adjacent to the first end to view the product. Since the components within the radiation detector have a maximum local operating temperature which may be less than the temperature of the process chamber, the temperature of the heat sink adjacent to the detector does not exceed the maximum operating temperature of the components within the detector.